Process for decomposing intermetallic compounds in metals

ABSTRACT

This invention relates to the purification of alloys and more particularly, to the decomposition of undesirable intermetallic compounds present in aluminum alloys. The invention is characterized by heating the impure alloy to a temperature above the melting temperature of the intermetallic compound, maintaining this temperature for a time sufficient to allow the intermetallic compound to dissociate, and subsequently cooling the alloy under conditions which minimize reformation of the dissociated compound. In the cooling phase of the invention, good results have been realized by utilizing an ice water quench.

United States Patent [72] Inventors Alex R. Valdo;

Freeman M. Sander-ford, both of Baton Rouge, La. [211 App]. No. 744,307 [22] Filed July 12, 1968 [45] Patented Oct. 26, 1971 [73] Assignee Ethyl Corporation New York, N.Y.

[54] PROCESS FOR DECOMPOSING INTERMETALLIC COMPOUNDS IN METALS 1111 Claims, No Drawings [52] 11.8. C1 75/.5 B, 2 64/ 13, 2 6 4/14 [51] Int. Cl ..B22d 2376i, 122f9l00 [50] Field of Search 75/.5; 264/5,13,14; 148/11.5

[5 6] References Cited UNITED STATES PATENTS 2,630,623 3/1953 Chisholm et a1. 75/15 2,869,?9'1' 1671 557 13111101 et al. 148/11.5 2,967,351 l/l961 Roberts et a1. 29/4205 3,226,267 12/1965 Foerster 148/115 3,252,841 5/1966 Foerster 148/115 3,282,745 11/1966 Foerster 148/115 3,291,654 12/1966 Foerster 148/115 3,307,978 3/1967 Foerster 143/115 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Att0rney-Donald L. Johnson ABSTltACTi invention riati the purification 1 loys and more particularly, to the decomposition of undesirable intermetallic compounds present in aluminum alloys. The invention is characterized by heating the impure alloy to a temperature above the melting temperature of the intermetallic compound, maintaining this temperature for a time suffcient to allow the intermetallic compound to dissociate, and subsequently cooling the alloy under conditions which minimize reformation of the dissociated compound. In the cooling phase of the invention, good results have been realized by utilizing an ice water quench.

PROCESS FOR DECOMPOSING RNTERMETALLIC COMPOUNDS IN METALS This invention is directed toward a novel technique for purification of alloys, and aluminum alloys in particular. Since many alloys contain undesirable impurities, it is frequently necessary to remove these impurities or at least reduce the quantity thereof before beneficial use can be made of the alloy. Consequently, there presently exists a need for a simple and inexpensive method for purifying alloys by reducing the quantity of undesirable impurities, such as intermetallic compounds, in these alloys.

Accordingly, it is an object of this invention to provide a process for reducing the content of undesirable intermetallic compounds in various alloys by thermally causing the compounds to dissociate. Another object of the invention is the purification of alloys which can be utilized commercially after removal of certain impurities. I in accordance with this invention, there is provided a process for reducing the content of one or more undesirable intermetallic compounds in an alloy by heating the alloy to a high enough temperature for a sufficiently long time to dissociate at least a portion of the intermetallic compound and then rapidly cooling the alloy to minimize reformation of the dissociated intermetallic compound. This heating causes a decomposition of the intermetallic compound or compounds to the constituent elements and the subsequent quenching retards reformation of these undesirable compounds by effecting a rapid temperature drop through the formation temperature of the compound. The invention is characterized by convenient flexibility in that substantially any alloy containing one or more undesirable intermetallic compounds can be purified provided the alloy is capable of undergoing peritectic decomposition. The term peritectic" may be defined as an isothermal reversible reaction in which a molten liquid phase reacts with a solid phase to produce another solid phase on cooling. This invention is directed toward the elimination of, or at least reduction in, the amount of certain intermetallic compounds present in this solid phase formed when the molten alloy is cooled. Thus, the term peritectic decomposition is used to denote the .process of reducing the quantity of certain intermetallic compounds originally present in the alloy by first heating the alloy to a temperature above the formation temperature of the intermetallics to dissociate these compounds and then rapidly quenching the alloy to minimize the reformation of the undesirable intermetallic compounds in the solid phase produced by cooling the molten alloy. In this manner it has been found that a solid phase may be formed which contains innocuous amounts of the compound sought to be eliminated.

Substantially any alloy may be easily purified according to the techniques of this invention, magnesium alloys, zinc alloys and the like being exemplary. Alloys containing aluminum are particularly well suited to upgrading by the instant process. Aluminum alloys which are readily susceptible to purification may have an aluminum content varying from a fraction of a percent up to 99 percent and higher. Aluminum alloys which are particularly susceptible to purification by this process are those containing from about 40 percent to about 90 percent aluminum by weight. The aluminum may be alloyed with substantially any element so long as one or more intermetallic compounds are present, the removal of which is desired. Exemplary of the metals and metalloids which may be present as intermetallics with aluminum in these alloys are iron, carbon, silver, boron, calcium, cobalt, chromium, manganese, nickel, titanium, uranium, vanadium, zinc, copper and silicon. Two or more of these, and other elements may be present in varying proportions with aluminum and may easily be removed from the alloy by the process of this invention. For example, common aluminum alloydntermetallic constituents which readily undergo peritectic decomposition and which are therefore susceptible of being removed by application of the inventive process are FeAl MnAl CuMgAl TiAl and FeSi Al Of the above enumerated intermetallic compounds which are frequently found to be undesirable constituents of aluminum alloys, compounds containing at least two of the elements, aluminum, silicon, titanium, iron, copper, magnesium and manganese are particularly significant because they are frequently found in useful alloys such as those obtained by the carbothermic reduction of clay. Alloys manufactured by the carbothcrmic reduction technique quite frequently contain, in addition to free aluminum, varying quantities of aluminum, silicon and iron as well as aluminum and titanium, in intermetallic form. Common intermetallic compounds containing these metal combinations are FeSi Ah and TiAl although it will be recognized that many other combinations are also frequently found. It has been found that by application of this invention such undesirable intermetallic compounds may be thermally decomposed, thereby enriching the aluminum content of the base aluminum alloy.

As above noted, a basic feature of the invention is the heating of an impure alloy to raise its temperature at least to, and preferably above, the melting point of the intermetallic compound which is to be decomposed in the alloy. Suitable temperatures which may be utilized in the invention are dependent upon the melting temperature of the intermetallic compound which is to be reduced in any given alloy. in general, however, temperatures in the range of from about 300 C. to about 1400 C. are high enough to melt intermetallic compounds which are susceptible to peritectic decomposition; For example, if decomposition of the intermetallic Fesi Al, is to be accomplished in an aluminum alloy containing by weight about 68 percent aluminum, 27 percent silicon, 3 percentiron and 2 percent titanium, the alloy should be heated to a temperature of at least 870 C. (which is the melting temperature of this intermetallic compound) before rapid cooling is effected. Similarly, if it is desirable to remove TiAl from the alloy having the same or a similar composition, a temperature of 1340 C. should be attained before quenching the alloy. in order to insure that substantially all the intermetallic compound is melted, it is preferable to heat the alloy to a temperature somewhat above the melting point of the intermetallic, e.g., up to about 20 percent above the melting point. of course, it is desirable to heat the alloy to as low a temperature as possible where a good intermetallic melt is achieved in order, upon cooling, to minimize the formation of other intermetallics than the one which is to be eliminated. That is, if some of the metallic, metalloid or intermetallic constituents of the alloy melt at a higher temperature than the intermetallic which is to be decomposed, the temperature to which the alloy should be heated is the melting temperature of the intermetallic to be removed, or perhaps slightly higher than this temperature. This procedure insures that the higher melting alloy constituent will not go into a solution and, upon cooling, reform into one or more additional undesirable compounds.

The length of time at which the alloy should remain at the intermetallic melting point is not critical, but it should be long enough to insure that all of the intermetallic compound or compounds to be decomposed are reduced to 'the molten state. Generally, a time interval of at least 1 minute is sufficient for alloys heated to temperatures within the range of from about 300 C. to about 1400 C. More preferably, the alloy should be held at the appropriate temperature from 2 to about 60 minutes in order to achieve the proper degree of intermetallic dissociation, and it has been found that 15 minutes is a sufficient dissociation time when the molten alloy has been subjected to stirring.

After the alloy from which undesirable intermetallic compounds are to be removed is heated to the appropriate tem' perature, it should be rapidly cooled in order to prevent the reformation of these compounds, as heretofore noted. A preferred cooling technique is the use of a rapid water quench, although air cooling has also been found effective under certain circumstances. Generally, a larger residue of the intermetallic compound sought to be removed has been found in the alloy subjected to air cooling as compared to an alloy of the same composition which has been subjected to water quenching. A more preferred technique for cooling the molten alloy is the use of an ice water quench to lower the temperature of the molten alloy as rapidly as possible. Of course, it will be recognized by those skilled in the art that many other quenching media and techniques may be utilized successfully in the cooling phase of this invention. In general, it is preferable to quench the alloy at a rate of from about 100 to about 400 per second, and most preferably, about 200 per second.

It has been found that the product alloy, that is, the alloy which has been purified by the technique of this invention, may be best commercially utilized in small particles. Of course, size reduction of the purified alloy may be undertaken after purification by conventional techniques such as crushing, grinding, and other methods well known to those skilled in the art. However, size reduction may be easily effected in the course of this process by sieving the molten alloy after heating it to the appropriate temperature. Thus, the alloy may be heated to the desired temperature and subsequently poured through a screen having a mesh size proportional to the size of the finished alloy particle desired, and when it is desired to utilize a liquid quench for cooling purposes, the molten alloy may be poured through a screen or similar size reducing apparatus directly into the quenching medium.

Accordingly, a preferred embodiment of the invention is characterized by heating for at least 1 minute, an alloy containing aluminum, silicon, iron and titanium, wherein the intermetallic compounds to be decomposed are composed of aluminum, silicon and iron and aluminum and titanium, respectively, to a temperature within the range of from about 300 C. to about l400 C. and then cooling the molten alloy by rapidly contacting it with a quenching medium. In a more preferred embodiment, the alloy contains by weight about 68 percent aluminum, 27 percent silicon, 3 percent iron and 2 percent titanium, the interrnetallic compounds to be decomposed as FeSi,Al and TiAl the alloy is heated in an inert atmosphere to a temperature of about 1340 C. and held at this temperature for about minutes, after which the alloy is cooled by rapidly dropping it in ice water. In a most preferred aspect of this embodiment of the invention the alloy is sieved, e.g., through a metal screen, before being contacted with the ice water or other quenching medium.

As heretofore mentioned, it is desirable to decompose certain undesirable intermetallic compounds present in alloys without simultaneously producing additional undesirable compounds. Consequently, the inventive process defines a technique for at least partially decomposing intermetallic compounds in a variety of metal and metalloid-containing alloys by first heating an alloy to a temperature at least as high as the melting temperature of the intermetallic compound to be decomposed but below the melting temperature of at least one of the metals or metalloids in the alloy, the yield of which is desired to increase. This temperature should be maintained for a time sufficiently long to thermally dissociate at least a portion of the intermetallic compound, and then the alloy should be rapidly quenched. This technique of controlled heating promotes the dissociation of undesirable intermetallics in the alloy to elemental, innocuous components. Subsequent rapid cooling of the alloy substantially preserves these components by retarding reformation of the undesirable intermetallics and minimizing the formation of other undesirable compounds containing the metal or metalloid, the presence of which is desired in elemental form in the alloy. in this aspect of the invention a preferred alloy for upgrading is composed of aluminum, silicon, iron and titanium and the intermetallic compounds to be decomposed contain at least two of the elements, aluminum, silicon, titanium, iron, copper, magnesium and manganese.

In a preferred embodiment of this aspect of the invention where the alloy contain the metalloid, silicon, it is desirable to heat the alloy to atemperature above the melting point of the 'intermetallic compound or compounds to be decomposed but below the melting temperature of the silicon, maintain this temperature for a time sufficiently long to dissociate the intermetallic constituents and then rapidly cool the alloy in order to increase the yield of silicon in the cooled alloy. As above noted, this technique retards reformation of the original intermetallic compounds in the alloy and also substantially reduces the amount of silicon-containing compound in the quenched alloy. In this manner, the quantity of elemental silicon or other element in a given alloy may be increased. As in previous embodiments of the invention, this temperature may generally be within the range of from about 300 C. to about 1400 C. and should be maintained for at least i minute. Further, the cooling stage of the process is preferably efiected by rapidly contacting the alloy with a quenching medium and in a preferred operating technique, the alloy is sieved after being heated to the appropriate temperature.

In a most preferred embodiment of the controlled heating aspect of the invention the alloy to be purified contains by weight about 68 percent aluminum, 27 percent silicon, 3 percent iron and 2 percent titanium and the intermetallic compounds to be decomposed are FeSi,Al and TiAl,. Further, the alloy is heated to a temperature of about l340 C. (which is below the melting point of silicon) in an inert atmosphere such as nitrogen or argon, where it is held for about 15 minutes, and the cooling phase is effected by rapidly quenching the alloy in ice water. In order to expedite future use the alloy is most preferably sieved before being subjected to ice water quenching.

This invention and the various embodiments thereof may be further understood by the following illustrative examples.

EXAMPLE 1 Approximately 500 grams of an alloy containing by weight 68 percent aluminum, 27 percent silicon, 3 percent iron and 2 percent titanium and having a FeSi,Al content of about 20 weight percent was heated to l,000 C. in a large crucible and held at this temperature for 30 minutes. The molten metal was then rapidly quenched in ice water by pouring the alloy directly into a water and ice mixture having a temperature of 2 C. After the quenching operation, the alloy was analyzed and found to contain 5.5 percent by weight of the FeSi,Al intermetallic compound.

EXAMPLE ll Three hundred forty-five grams of an alloy having the same composition as that used in Example I was heated in a crucible to a temperature of l,000 C. where it was held for 15 minutes and vigorously stirred. The molten alloy was then poured through a stainless steel screen into ice water, the temperature of which was 2 C. Small particles of alloy were observed to form in the ice water. The analysis of FeSi,Al present in the alloy after quenching was found to be 9 percent by weight.

EXAMPLE Ill Ten grams of silicon alloy containing 75 percent by weight of silicon, 5 percent by weight of aluminum, 19 percent by weight of FcSi Ab and 1 percent by weight of TiAl was heated in a crucible to 1200" C. and held at that temperature for 15 minutes. The alloy was then allowed to cool in air. The cooled alloy was subjected to X-ray analysis which showed on a weight basis, 82.4 percent silicon, 1.6 percent aluminum, 7.4

percent FeSi,Al and the presence of two unknown intermetallic compounds.

EXAMPLE IV Twenty-five grams of an alloy containing by weight 73 percent silicon, 5 percent aluminum, 22 percent FeSi,Al, and a trace of TiAl was heated in an argon atmosphere to l;000 C., held for 15 minutes and subsequently quenched in ice water. Upon X-ray analysis, the FeSi Al content was found to be 2.2 percent by weight and there appeared to be no trace of TiAl, left in the alloy.

EXAMPLE V.

Three gram samples of aluminum alloy containing by weight 27.4 percent aluminum, 50.2 percent silicon, 8 percent iron (FeSi Al was present in an amount equal to 19 percent by weight) and 0.4 percent titanium were heated to temperatures of 800 C., 900 C. and 1000 C., respectively. The alloy samples were held at these respective temperatures for minutes and were then rapidly quenched in ice water. X-ray analysis of the three samples showed the FeSi AI, content of the samples by weight as follows: 800 C.-l4.9 percent; 900 C.-- 14.9 percent; 1000 C.- 1.9 percent.

Example V clearly illustrates that for best results the alloy to be purified should be heated to a temperature significantly above the melting point of the intermetallic compound to be decomposed. The melting point of FeSi Al as previously noted, is about 870 C., and the results of Example V show that much more FeSi AL, impurity is removed by quenching from a temperature of 1000 C. (well above 870 C.) than from temperatures below and even slightly above the intermetallic compound melting point.

From an analysis of the preceding examples and embodiments it is apparent that the invention provides an easy and economical method for decomposing a quantity of undesirable intermetallic compound in virtually any alloy capable of undergoing peritectic decomposition. Operation procedures are simple and necessary equipment is not expensive and is readily available. Accordingly, the invention contributes to the art of alloy purification.

What is claimed is:

I. A process for at least partially decomposing intermetallic compounds in metal and metalloid-containing alloys which comprises, in combination, the steps of heating the alloy to a temperature at least as high as the melting temperature of the intermetallic compound but below the melting temperature of at least one of the metals or metalloids in the alloy for a period of time sufiiciently long to thermally dissociate at least a portion of the intermetallic compound and then rapidly cooling the alloy to minimize reformation of the intermetallic compound.

2. The process of claim ll wherein:

a. said alloy contains aluminum, silicon, iron and titanium,

and

b. said intermetallic compounds contain at least two of the elements, aluminum, silicon, titanium, iron copper, magnesium and manganese.

3. The process of claim 2 wherein said at least one of the metals is silicon.

41. The process of claim ll wherein said alloy is heated to a temperature within the range of from about 300 C. to about l400 C. and held at this temperature for at least 1 minute.

5. The process of claim ll wherein said cooling is effected by rapidly contacting said alloy with a quenching medium.

6. The process of claim 1 wherein said alloy is sieved after said heating.

7. The process of claim 1 wherein:

a. said alloy contains by weight about 68 percent aluminum,

27 percent silicon, 3 percent iron and 2 percent titanium,

b. said intermetallic compounds are FeSi Al and TiAl;,,

c. said at least one of the metals is silicon,

d. said alloy is heated in an inert atmosphere to a temperature of about 1,340" C. and held at this temperature for about 15 minutes, and

e. said cooling is effected by rapidly contacting said alloy with ice water.

8. The process of claim 7 wherein said alloy is sieved before being contacted with said ice water.

9. The process of claim 1 wherein said alloy is an aluminum alloy.

10. The process of claim 1 wherein said intermetallic compound contains at least two of the elements, aluminum, iron, carbon, silver, boron, calcium, cobalt, chromium, manganese, nickel, titanium, uranium, vanadium, zinc, copper and silicon.

11. The process of claim 1 wherein said intermetallic compound is a first compound composed of aluminum, silicon and iron and a second compound composed of aluminum and titanium. 

2. The process of claim 1 wherein: a. said alloy contains aluminum, silicon, iron and titanium, and b. said intermetallic compounds contain at least two of the elements, aluminum, silicon, titanium, iron copper, magnesium and manganese.
 3. The process of claim 2 wherein said at least one of the metals is silicon.
 4. The process of claim 1 wherein said alloy is heated to a temperature within the range of from about 300* C. to about 1400* C. and held at this temperature for at least 1 minute.
 5. The process of claim 1 wherein said cooling is effected by rapidly contacting said alloy with a quenching medium.
 6. The process of claim 1 wherein said alloy is sieved after said heating.
 7. The process of claim 1 wherein: a. said alloy contains by weight about 68 percent aluminum, 27 percent silicon, 3 percent iron and 2 percent titanium, b. said intermetallic compounds are FeSi2Al4 and TiAl3, c. said at least one of the metals is silicon, d. said alloy is heated in an inert atmosphere to a temperature of about 1,340* C. and held at this temperature for about 15 minutes, and e. said cooling is effected by rapidly contacting said alloy with ice water.
 8. The process of claim 7 wherein said alloy is sieved before being contacted with said ice water.
 9. The process of claim 1 wherein said alloy is an aluminum alloy.
 10. The process of claim 1 wherein said intermetallic compound contains at least two of the elements, aluminum, iron, carbon, silver, boron, calcium, cobalt, chromium, manganese, nickel, titanium, uranium, vanadium, zinc, copper and silicon.
 11. The process of claim 1 wherein said intermetallic compound is a first compound composed of aluminum, silicon and iron and a second compound composed of aluminum and titanium. 